Fertilization is a cell biological process with important medical, social and economic implications. The underlying cell biological functions of gamete interactions are conserved in all cells regardless of their somatic or germ-line origins. Despite intense study, sperm-egg interactions are still poorly understood at the molecular level. Most previous work on fertilization has relied on biochemical and immunological approaches while a genetic approach has been lacking. We are pioneering the use of the nematode worm Caenorhabditis elegans for addressing the molecular mechanisms of sperm-egg interactions. The powerful tools of classical and molecular genetics developed for the worm are not available or are very difficult to utilize in the other organisms traditionally used for studying fertilization. The reproductive biology of C. elegans facilitates the identification of mutations that affect sperm and no other cells. These mutations provide a unique opportunity to define sperm components required for sperm-egg interactions. Worms with mutations in two interacting genes, spe-9 and spe-13, produce spermatozoa with wild-type morphology and motility that cannot fertilize oocytes even after contact between gametes. Therefore, disruption of spe-9 or spe-13 function affects either gamete recognition, adhesion, signaling and/or fusion. The spe-9 gene encodes a sperm transmembrane protein with an extracellular domain that contains ten epidermal growth factor (EGF)-like repeats. A common feature of proteins that include EGF-like motifs is their involvement in extracellular functions such as adhesive and ligand-receptor interactions. These results are consistent with the hypothesis that SPE-9 functions in the specialized cell-cell interactions required for fertilization. In order to more precisely define the role of SPE-9 during fertilization, we will determine its localization in sperm and reveal the cellular regions important for gamete interactions. Furthermore, we will investigate how different domains of the protein contribute to its biological function. We will clone the spe-13 gene and analyze its gene product. This information will be useful in formulating a molecular model concerning the role of SPE-13 during fertilization and possible interactions with SPE-9. We will initiate the characterization of several new genes that when mutated appear to phenocopy the spe-9 and spe-13 mutations. Such genes are expected to encode additional sperm components required for fertilization. Finally, we will conduct genetic screens to identify the oocyte receptor for SPE-9 and other genes required for sperm-egg interactions. This work will provide new insight into cell-cell interactions, conception and complement studies of fertilization in other organisms.